And Eydroceemical Investigation Op Baürü- and Botdcatu Groündwaters, Parana Basin, Brazil
Total Page：16
File Type：pdf, Size：1020Kb
.88. ENVIRONKENTAL ISOTOPE- AND EYDROCEEMICAL INVESTIGATION OP BAÜRÜ- AND BOTDCATU GROÜNDWATERS, PARANA BASIN, BRAZIL (report covering 1989 and 1990) Kisunelmann, A. Universidade de Sao Paulo, Instituto de Geociencias Centro de Pesquisas de Aquas Subterrâneas June, 19-91, Sao Pauic .89. Content of this report 1. Introduction 2. Geological and hydrogeological situation of the Parana- Basin 2.1. The Bauru Aquifer 2.2. The Serra Geral Basalt 2.3. The Bctucatu Aquifer 3. Isotope methods used for groundwater-investigation 3.1. 2H- and 180-contents in groundwaters 3.2. i4c- and 13C-contents in groundwaters 3.3. 3He/4He-contents in groundwaters 3.4. 4He-excess data for groundwater-investigation 4. Sampling and analytical methods 4.1. Geochenical and isotope data in the core-samples from the well Presidente Prudente 2 4.2. 2H- and 18O-data of Bauru and Botucatu groundwaters 4.3. 3H-contents in Bauru- and Botucatu groundwaters 4.4. *3c- and 14C-contents of Bauru and Botucatu groundwaters 4.5. 3He/4He- and He-excess data in deep groundwaters from Botucatu 5. Hydrochenical analyses of Bauru- and Botucatu groundwaters 6. Summary and Advances 7. Future activities 8. 1990-1991 Timeschedule of the Bauru-/Botucatu-Project 9. Cooperating Institutions 10. Tables and figures .90. 1. Introduction A combined hydrogeochemical and environmental isotope investigation is being carried out in the Bauru- and Botucatu groondwaters. Based on former isotope data, the 2H/18O-, 3H-, 13C-, l*c- and 3He/*He-contents of representative groundwaters out from the Bauru- and Botucatu-aquifer are now investigated to reveal a possible interaction of the groundvater systems and to correlate 14C-groundwater residence tines with groundvater "ages'* derived from other isotope methods. The area of investigation includes the southwestern part of the State of Sao Paulo and the northern Parana and follows a profile between the border of the Parana Basin to its central part. In the east it covers the city of Analandia and in the west it reaches the city of Presidente Epitacio, including the Londrina region. Continuing former isotope investigations and in the context of a more detailed isotope investigation, groundwaters were selected in view of different drilling and casing depths to represent Bauru- and Botucatu groundwaters. In 1989 and 1990, two groundwater sampling campains followed a trajectory from Sao Paulo to the city of Epitacio. Sampling drilling cuts from the new drilled well in Pres. Prudente first time enabled to check for 513C-contents and geochemical composition of this aquifer. Beside the aspect, that i13C-rock data are essential for evaluating carbon isotope hydrochemistry of the inorganic carbon dissolved in groundwater, geochemical data are of interest when interpreting hydrochemical contents of the groundwaters. Results of hydrogeochemical- and isotope analyses obtained in the 1989 and 1990 campains are given in chapter 10 and a first interpretation of these new data is presented in this report. 2. Geological and hydroqeolooical situation of the Parana- Basin The main aquifers in the brazilian part of the Parana Basin are the Bauru-, the Serra Geral- and the Botucatu-Piramboia aquifer systems. .91. 2.1 The Br.uru Aquifer The Bauru aquifer system covers 104 000 km2 in the State of Sao Paulo. It is an unconfined aquifer which is formed from cretaceous sandstones. The average thickness of this aquifer is around 100m but increases towards the central part of the area of investigation. The Bauru aquifer can be subdivided into four formations: Marilia, Adamantina, Santo Anastácio and Caiua. Because the Caiua is the most effective aquifer, its exploitation through wells with depth between 40m and 200m is intensive. In parallel to this, and due to industrial resp. agricultural activities on its surface, the aquifer is also vulnerable to pollutant input. In consequence to the use of fertilizers in agriculture, N03- contents up to 50 mg/1 are found in Bauru-groundwaters. Beside fertilization, also different industrial activities represent a source for Cr-, Ba-, Hg- and Cd-contamination of Bauru- groundwaters . 2.2 The Serra Geral Basalt The Serra Geral Basalt is a fractured aquifer system with very inhoinogenous hydraulic characteristics. In the area of investigation only few wells are exploiting this aquifer, as its groundwater yield is not very high. In the first sampling campain two samples of drilling cuts could be obtained and a geochemical analysis (performed at University of Góttingen, FRG) revealed the Serra Geral Basalt being a U- and Th-low, SiO2-dominated and carbonate-low basalt with Fe03- and Al203-contents of 5 and 10 % (weight-%) (comp. tab.2). 2.3 The Botucatu Aquifer The most important aquifer system of the Parana Basin is the Botucatu- (Piramboia) sandstone. It covers around 800 000 km2 on Brazilian territory and, from the North to the South, belongs to the states Mato Grosso do sul, Minas Gerais, Sao Paulo, Parana, Santa Catarina and Rio Grande do Sul. .92. The Botucatu aquifer system is constituted by the two sedimentary formations Piraaboia and Botucatu. The Piranboia formation consists of fluvial sandstones. In contrast to the Piraaboia, the Botucatu formation consists of sandstones of eolic origin. The Piramboia and the Botucatu formations are intercalated between the Permian layers of the Passadois Group and the basalt flows of the Serra Geral Formation. The Botucatu-aquifer is unconfined at the borders of the basin. The unconfined part is only some 10% of its area. The Botucatu aquifer is confined from the basalt flows of the Serra Geral Formation. The thickness of the covering basalt may reach about 2000 a in the center of the Parana-Basin. Hydrogeological studies revealed groundwater recharge rates up to 160 x 10 mvyear f°r Botucatu-groundwaters. The recharge of the aquifer is mainly due to infiltration of rain water in the outcrop areas. Froa the border of the basin towards its central part, groundwater flow direction follows the direction of the diving of the geological units. The Botucatu aquifer is exploited through wells with depths between 150 a and some 3000 m and exploitation pumping rates range from 100 a3/h to 1000 m3/h. In consequence to the flow rates and drilling depths, Botucatu groundwaters in the central part of the Parana Basin are geothermal and represent an important geothermal potential. In view of the fact that the Bauru- and Botucatu aquifers, from its hydrogeochemical and bacteriological status, are suited for the population's, the industry's and the agriculture's water- supply, a combined hydrogeochemical and environmental isotope investigation is carried out to evaluate possible interaction of the groundwater systems on the basis of environmental isotope methods. 3. Isotope methods used for oroundvater investigation In this project the application of different isotope methods will be performed to cover the groundwater-ages and to check for deep reaching groundwater-mixing systems. As could be seen even from 14C-data in geothermal groundwater in Pres. Prudente, recent water-input into a deep and geothermal groundwater mixing system could be found and apparently is due to deep reaching fractures in the Serra Geral Basalt and to overexploitation of this well. .93. As initial contents of 14C can vary within a wide range in the aquifers investigated, a separate attempt to evaluate C-isotope evolution on the basis of 13C-data is Bade: beside Measuring 13C-contents in the dissolved inorganic carbon (DIC) of the groundwater, a «ore detailed approach to c-isotope evolution Bust include the 13C-contents in the aquifer materials. Consequently we started to measure 13C-contents in drilling cuts from different depths of Pres. Prudente 2. As *4C-dating of geotherval groundwaters fails even when correcting with 13C-contents in DIC and host-rock, enforced C-solution and enforced C-isotope exchange between the dissolved inorganic carbon and the carbon in the host-rock takes place and 14C-data consequently only can serve as rough indicators for recent water input. To date these groundwaters, sanples were taken for 3He/4He- and *He-excess-Beasureaents. It is also foreseen in this context to use noble gas aethods to determine recharge-paleotemperatures of the very old groundwaters. In the following chapters a short desription of the different isotope methods will be given. 2H- and 180-contents in qroundwaters The measurement of 2H- and *°0-contents in groundwater reveals its meteoric/non-meteoric origin and, in cases of changing recnarge climate (paleotemperatures), 2H- and 18O-data can give hints to very old groundwaters (paleo-ef feet). When applying the noble gas method to reveal recharge temperatures, these ll2H-/180-paleogroundwatersM would be of special interest. 3.2. 14c- and 13C-contents in aroundwater The 14C-method to determine mean residence times of groundwater covers the dating range between some 2000 and 30 000 years. Because the 14C-content is determined in the inorganic carbon dissolved in groundwater, the initial 14C-content of the system groundwater/aquifer nust be evaluated using the "Chemical Mixing-1* and/or the "C-Isotope Exchange Model". Both models simulate the geochemical evolution of the 14C-content following the groundwater pathway from the infiltration- or recharge area to the sampling site. .94. The models correct for the C-isotope interaction -and depletion of the 14C-signal- between the DIC in the groundwater and the C-component of the rock matrix. The rock carbonate -as one component of C-isotope evolution of DIC in groundvater- is free from 14C but shows typical 13C-contents. 13 As the C-content in atmosperic C02 is near some -24 6-0/00 13C vs. an international standard (PDB), the 13C-content of DIC approaches the 13C-content of the rock. Depending on the formation of the C-component of the rock, the 13C-content can vary in the range +0.5 4-0/00 13C for e.g.